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Events on Thursday, February 13th, 2014

NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Hadronic Interaction Models and the Interpretation of Astroparticle Data
Time: 2:30 pm
Place: 4274 Chamberlin Hall
Speaker: Dimitra Atri, Blue Marble Space Institute of Science
Abstract: The interpretation of astroparticle data from both ground and space-based instruments depends strongly on the hadronic interaction models underpinning the emission mechanism. I will demonstrate the model dependence using recent observations from the Ooty air shower array and Fermi LAT. I will also discuss how the choice of hadronic interaction models could affect the interpretation of neutrino and gamma ray measurements with IceCube and CTA.
Host: Justin Vandenbroucke
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Astronomy Colloquium
Asymmetries in Protoplanetary Transition Disks
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Hui Li, Los Alamos National Laboratory
Abstract: Particle energization in astrophysical systems is a challenging yet fascinating subject. Observations from radio to gamma-rays of AGN jets, supernova remnants, pulsars, gamma-ray bursts, etc., have provided important clues. In this talk, we explore the energy conversion processes in a parameter regime when the magnetic energy density overwhelms the particle energy density (including its rest mass energy density). Such a condition has been discussed in the context of AGN jets and pulsars. We present two-dimensional and three-dimensional relativistic, full kinetic simulations that show fast magnetic reconnection can occur in highly magnetized plasmas, with the magnetization parameter ranging from unity to 1600. The fast magnetic dissipation leads to a fast, nonthermal particle acceleration, yielding a power law distribution with a hard spectrum. Detailed analyses show that the acceleration in the power-law range is mainly by a Fermi-like mechanism in the relativistic flows generated by reconnection. Implications for observations will be discussed.
Host: Professor and Dept Chair Ellen Zweibel
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Faculty Candidate Seminar
Engineering New Electronic States in Graphene Heterostructures: Massive Dirac Fermions, Hofstadter’s Butterfly and the Quantum Spin Hall Effect
Time: 4:00 pm
Place: 4274 Chamberlin Hall
Speaker: Benjamin Hunt, MIT
Abstract: Van der Waals heterostructures represent a new and surprising direction in nanoscale device engineering: we stack isolated two-dimensional crystals to create layered structures with atomic precision. The layer-by-layer assembly allows us to introduce a new design parameter - the interlayer twist angle - which can have profound consequences for the engineering of electronic states based on tunable interactions between adjacent layers. In this talk, I will discuss recent experiments at MIT in which we have used a hexagonal boron nitride (hBN) layer to modify the electronic bands of monolayer graphene in a van der Waals heterostructure, inducing a sizable bandgap at the charge neutrality point and imparting a mass to the normally massless Dirac charge carriers. The bandgap occurs only in samples in which the twist angle between the graphene and hBN crystals is small, resulting in a long-wavelength moirA(c) that acts as a superlattice potential; by adjusting the twist angle the bandgap can be tuned. The moirA(c) superlattice potential also allows us to study the problem of a charged particle in a periodic potential and magnetic field aEuro" the so-called Hofstadter problem aEuro" whose theoretical solution exhibits a rare instance of fractal behavior in a quantum-mechanical energy spectrum.

I will also discuss our recent studies of weakly-coupled graphene-hBN heterostructures in which massless Dirac fermions in graphene exhibit a quantum spin Hall effect, a fascinating example of a aEurooesymmetry-protected topological phaseaEuro of which the more familiar contemporary examples are the edge and surface states of the topological insulators.
Host: Mark Eriksson/Sridhara Dasu
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